Electrical appliances such as a digital camera set require various power supply voltages in order to drive a motor, a memory, a speaker, a backlight, and so forth. A synchronous rectifying type DC-DC converter is used to convert an input voltage to a desired voltage. In a present market of digital cameras, for example, low-power-consumption and long-time operation are required. In order to achieve the long-time operation, improvement of power efficiency is required for DC-DC converters. As a result, although being not considered in the past, high power efficiency is now required even in a case of no load or a light load.
One related art is a switching power supply unit disclosed in Patent Literature 1 (JP 2010-239778A). FIG. 1 is a configuration diagram showing a step-down synchronous rectifying type switching power supply unit disclosed in Patent Literature 1. The switching power supply unit comprises a main transistor MP11, a synchronous rectifying transistor MN11, an inductor L1, a capacitor C1, a power supply control circuit 114, a P-channel driver 115, and an N-channel driver 116A. When the main transistor MP11 is turned on by the P-channel driver 115 (the synchronous rectifying transistor MN11 is turned off) in the switching power supply unit, a current flows from the power supply 111 to the capacitor C1 through the main transistor MP11 and the inductor L1, to charge the capacitor C1. When the synchronous rectifying transistor MN11 is turned on by the N-channel driver 116A (the main transistor MP11 is turned off), a current flows from the synchronous rectifying transistor MN11 to the capacitor C1 through the inductor L1 due to energy accumulated in the inductor L1, to charge the capacitor C1. In the case of the latter, when the load current is small, there is a case that a direction of the current flowing through the inductor L1 is reversed (flow backward). Due to reverse current, energy is lost and power efficiency is lowered. For this reason, change in voltage V_LX1 at a node LX1 is detected by the N-channel driver 116A, and the synchronous rectifying transistor MN11 is turned off when the direction of the current is reversed.
FIG. 2 shows operation waveforms of the N-channel driver 116A. When a power supply control circuit 114 switches a control signal PRDRV_N from “L” to “H” (the control signal PRDRV_P changes from “L” (a low level) to “H” (a high level), so that the main transistor MP11 is turned off), a drive signal DRV_N changes from “L” to “H” and the synchronous rectifying transistor MN11 changes from the off state to the on state, thereby the current flows from the synchronous rectifying transistor MN11 to the capacitor C1 through the inductor L1. As a result, the voltage V_LX1 at the node LX1 has a negative voltage lower than ground voltage (GND). After that, the current flowing through the inductor L1 decreases and the voltage at the node LX1 increases as time passes. When the current flowing through the inductor L1 becomes zero, the voltage V_LX1 at the node LX1 also becomes zero. After that, the reverse current starts to flow from the inductor L1 to the synchronous rectifying transistor MN11. In order to prevent the reverse current, the N-channel driver 116A controls the synchronous rectifying transistor MN11 to be set to the off state when the voltage V_LX1 becomes zero. As a result, lowering of power efficiency due to the reverse current can be prevented.
According to the switching supply unit disclosed in Patent Literature 1, lowering of power efficiency is prevented by preventing generation of the reverse current in a case of a light load. However, the states of the main transistor MP11 and the synchronous rectifying transistor are switched by use of PWM (Pulse Width Modulation) control. An on time period of the main transistor MP11 becomes short in a case of the light load. As a result, an increase in the output voltage due to one switching is reduced. Constant switching is necessary in order to keep output voltage. Constant switching, however, leads to an increase in an operation rate of a circuit. Power efficiency is calculated as a ratio of output power to input power (output power/input power). The input power is calculated by multiplying the entire circuit operation current by input voltage. An increase in an operation rate of a circuit raises a value of input power, causing lowering of power efficiency.